This invention relates to gamma cameras used for medical nuclear imaging and, in particular, to gamma cameras with automatic adjustment of the detected energy spectrum.
Gamma cameras are widely used to make images of the interior of the body by detecting emissions of radioisotopes injected into a patient""s body. The detector or detectors of a gamma camera are highly sensitive to radioisotope emissions, as they must be able to detect the energy level of an emission event as well as the precise location of the event. Consequently virtually all gamma cameras have procedures for calibrating the detectors. Usually this is done with a source of radiation that floods the detectors with a known level of radiation. Detector gain circuits and/or correction tables are then adjusted to align the outputs of the photomultiplier tubes (PMTs) to the known radiation us level.
However, the characteristics of the detectors can drift continuously even during the course of an imaging procedure. These variations are due to factors such as temperature changes, magnetic field variation, and PMT drift. One approach to dealing with this continuous drift is described in U.S. Pat. No. 5,677,536, where histograms of each PMT are sampled each time the number of scintillation events reaches a predetermined count, and the PMT gain or calibration tables for each PMT is adjusted following each sampling time. These corrections are performed xe2x80x9con the flyxe2x80x9d during operation of the camera, and run continuously while the camera is being used. A less complex approach is to analyze the histogram for the current location of the peak energy value, then adjust the energy window for that peak to be centered around the current peak energy value. However, these approaches are very processing intensive, imposing significant operating overhead on the camera system, and the window adjustment approach is distressing to clinicians who expect a physical constant to remain so. Accordingly it is desirable to be able to adjust a gamma camera for drift without the foregoing drawbacks.
In accordance with the principles of the present invention, the detector of a gamma camera is automatically adjusted for drift and other factors producing an unwanted shift of the energy spectrum peak. A histogram of event data is continuously acquired and updated while the camera is in use. Upon receipt of a command to perform autopeaking, recently acquired event data is analyzed to determine if a valid energy histogram is present. This analysis may comprise steps such as determining if a minimum number of counts have been recorded, examining the signal-to-noise ratio of the photopeak, and analyzing the ratio of counts inside an energy window to the counts outside the window. The system is informed of the identity of the isotopes being used so that a check can be made of the acquired data to expected values. When a valid histogram is present, the system determines the peak value of the photopeak, and then computes a gain correction factor based upon the relation of the measured peak to the theoretical peak value for the isotopes present. The gain correction factor is applied to all energy values, thereby providing a correction for all photopeaks of all isotopes present.